1. Field of the Invention
The present invention relates to an optical encoder to be used for displacement measurement and angle measurement.
2. Description of the Related Art
An optical encoder includes a main scale provided with a first optical grating; an index scale facing the main scale and provided with a second optical grating; a light source illuminating the main scale; and photoreceptors subjected to light that is transmitted or reflected off the first optical grating of the main scale and is further transmitted through the second optical grating of the index scale.
In optical encoders of this type, an optical encoder having an index scale combined with photoreceptor arrays is proposed in Japanese Patent Publication No. 6-56304. An encoder with such a structure is also disclosed in Japanese Patent Laid-Open No. 2003-161645 by the present inventors.
An encoder with the above-described structure is called an incremental encoder, which is capable of detecting the amount of travel of the scale based on fluctuations in pulse.
A disadvantage of such an incremental encoder is that it requires an additional sensor for detecting the absolute position, as the absolute position of the rotation angle is not known. A solution to this is disclosed in Japanese Patent Laid-Open No. 10-318790 (corresponding to U.S. Pat. No. 5,929,789).
This document discloses an incremental transmission encoder in which, as shown in FIG. 13, slits of varying transmittance are arranged in a scale 1 to detect the absolute position. For example, the transmittance of a slit 2a is set to 1, and the transmittance of the other slits gradually decreases in the order of a slit 2b, a slit 2c, and a slit 2d. 
FIG. 14 shows signal fluctuations that can be obtained, using the scale 1, when a portion with the slits of varying transmittance passes through a sensor. Since the amplitude of two-phase analog signals Oa and Ob obtained from the sensor is reduced as the transmittance of the slits gradually decreases, the absolute position can be determined by detecting the change in amplitude.
In the detection of signal amplitude to determine the absolute position using the known encoder described above, there is a need to carry out sampling at intervals well shorter than one period of the encoder signal to be obtained, in order to determine the voltage at the peak and valley of the signal obtained. This processing requires a large-scale circuit, such as a high-speed A/D converter.
Moreover, a portion with varying transmittance needs to be created, on the scale, with a high degree of accuracy. There is a possibility that considerable variations in actual amplitude change may result.